1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to structures for introducing pretilt to liquid crystal materials for the displays.
2. Description of the Related Art
Flat panel displays have become increasingly important in the computer industry and in other industries where the display of information is important. These types of displays provide unique opportunities for lowering the weight, size, power consumption and eventually the cost of displaying information.
Liquid crystal displays seem to hold the most promise as the technology which will eventually be utilized in almost all practical flat panel displays. Considerable success has been achieved in small size color televisions and in monochrome flat panel displays as well as larger sizes used in color notebook, laptop computers or desk top monitors. However, unlike the cathode ray tube display, which exhibits good viewing quality from a variety of angles, conventional liquid crystal displays suffer from a loss of contrast or contrast reversal when viewed at an angle beyond about 15 degrees from the normal to the plane of the display. This is due to the interaction of light with the molecules of the liquid crystal material in the liquid crystal display cells. Light traveling through these display cells at other than a normal angle of incidence interacts with the liquid crystal display molecules in a manner different from that of light traveling with normal incidence. The contrast between a light transmissive (white) state and a non-transmissive state (black) at other than the normal angle is decreased, thus making such displays less desirable for use in many applications, such as flat panel television screens and large computer screens.
There have been various attempts to solve this problem. One method is discussed in U.S. Pat. No. 5,309,264, commonly assigned to the assignee of the present invention, wherein a pattern of openings is formed in the common electrode. Such openings cause the display elements of the display to have more than one liquid crystal domain. This is an elegant approach; however, to provide sufficient optical performance, the width of such openings is required to be about twice that of the cell gap or larger. Importantly, for high resolution displays (such as .gtoreq.120 pixels per inch), the width of a given display element may be on the order of twice or more that of the cell gap. In this case, this method becomes ineffective.
Another approach to solving this problem is to use an in-plane switching LC mode. This has the disadvantage that closely spaced electrodes are needed to provide the required lateral electric fields. The needed electrodes reduce the yield, aperture ratio, and scale poorly to higher resolution displays.
There have been various attempts to provide liquid crystal displays with a wide viewing angle without degrading the contrast ratio or brightness. The wide viewing angle must also be provided at low cost. One method is discussed in application Ser. No. 08/960,826, which is commonly assigned to the assignee of the present invention and incorporated herein by reference.
Referring to FIG. 1, a top view of a conventional liquid crystal display device 30 is shown wherein a pixel electrode 26 is formed below the pixels (6 are shown) of display 30. The pixels are formed between gate lines 32 (3 shown) and data lines 31 (4 shown).
FIG. 2 illustrates a partial cross-section of the conventional liquid crystal display device 30 of FIG. 1. Device 30 includes a first substrate 25 and a second substrate 27 formed of a transparent material such as glass. The two substrates are arranged so as to be parallel to one another with a high degree of precision. Typically, the substrates 25, 27 are separated from one another by a distance of approximately one to twenty microns, and are sealed at their edges (not shown) so as to define a closed interior space there between. First substrate 25 has deposited thereon an array of pixel electrodes 26 which define pixels of the liquid crystal display. Also formed on substrate 25, in selected areas not having electrode films deposited thereon, are semiconductor devices such as diodes or thin film transistors (TFTs) 37. As is well known in the there are one or more TFTs 37 for each pixel. TFTs 37 are each controlled by a conductive gate line 32 (not shown) and a conductive data line 31, which are typically deposited on substrate 25 in a manner so as not to be electrically connected to electrodes 26 except that the source of each TFT 37 is electrically connected to one respective electrode 26. Gate lines 32 (not shown) and data lines 31 are also electrically insulated from one another at crossover regions. The second substrate 27 typically has deposited thereon a color matrix layer 23. The color matrix layer 23 typically has a black matrix material 23-1 interleaved with R, G, or B color matrix material 23-2 and is frequently underneath the R, G, B color matrix material. The black matrix material 23-1 is disposed opposite the TFTs 37, data line 31 and gate line 32 (not shown) to block the devices from ambient incident light and prevent light leakage outside the pixel area. The color matrix material 23-2 is disposed opposite the pixel electrode 26. In addition, a continuous electrode 28 is typically formed on the color matrix layer 23 or a transparent overcoat layer. The continuous electrode 28 is preferably formed of a thin layer of a conductive material, such as indium tin oxide (ITO) or other suitable material.
A liquid crystal material 36 fills the space between substrates 25 and 27. The nature of the material depends on the mode of operation of liquid crystal display 30.
The interior surfaces of the liquid crystal display may be coated with respective alignment layers 38 and 40 to provide boundary conditions for the molecules of liquid crystal material 36.
The exterior surfaces of substrates 25 and 27 may have respective optical compensating films 42 and 44 disposed thereon. Finally, respective polarizing films 46 and 48 may be applied over compensation films 42 and 44 (if compensating films are used), respectively, or applied over substrate 25 and 27 (if compensating films are not used), respectively.
Conventional liquid crystal displays of the type illustrated in FIG. 2 are illuminated by a light source (not shown) that is located below the panel (the substrate 25 side) and viewed from above the panel (the substrate 27 side).
Liquid crystal cells typically are characterized by a pixel area and cell gap. The pixel area of a given cell is defined by the width W and the length L of the pixel electrode pattern of the cell as illustrated in FIG. 1. In addition, the cell gap is defined by the distance between the alignment layers 38, 40 as shown in FIG. 2.
As illustrated in FIG. 3A, in the case of a homeotropic type LCD, liquid crystal (LC) molecules near the electrodes 26 and 28 are aligned so that the long axes of the LC molecules are almost perpendicular to the electrode surfaces when no electric field is applied between the pixel electrode 26 and the electrode 28. The molecules have a small pretilt angle, typically one to fifteen degrees of tilt, away from the substrate normal. As illustrated in FIG. 3B, when an electric field is applied between the electrodes 26 and 28 of the homeotropic liquid crystal display cell, the molecules are caused to be oriented in a direction substantially perpendicular to the electric field.
Homeotropic liquid crystal cells require a liquid crystal material that exhibits negative dielectric anisotropy, such as ZLI-4788, ZLI-2857 or 95-465MLC manufactured by E. Merck Darmstadt of Germany and available in the United States through EM Industries. The alignment of the LC molecules of the homeotropic cells is typically provided by rubbing alignment layers 38, 40. An example of such rubbing steps is described in K. W. Lee et al., "Microscopic Molecular Reorientation of Alignment Layer Polymer Surfaces Induced by Rubbing and its Effects on LC Pretilt Angles", Macromolecules, Vol. 29, Number 27, 1996, pages 8894-8899. The alignment layers may be formed, for example, from polyimide SE-1211 manufactured by Nissan.
As is well known in the art, homeotropic cells typically use a compensating film to reduce dark state light leakage for light that travels through the liquid crystal display panel in a direction other than perpendicular to the substrates. For best results, the product of the thickness of the liquid crystal material layer in the liquid crystal display cell and the difference between the extraordinary and ordinary indexes of refraction for the liquid crystal display material is equal to or close to the product of the total thickness of the compensating films and the difference between the ordinary and extraordinary indexes of refraction of the compensating film. It is understood by those skilled in the art that other cell configurations may be used.
At least one thin wall (ridge 10, FIG. 4) may be formed on either the pixel electrode 26 or the electrode 28 of the homeotropic liquid crystal display cell. Wall or ridge 10 is preferably formed from a polymeric material or other dielectric material. In the case that the thin wall is formed on the pixel electrode 26, the alignment layer 38 is formed on both the pixel electrode 26 and the thin wall(s). In the case that the thin wall is formed on the electrode 28, the alignment layer 40 is formed on both the electrode 28 and the thin wall(s). The wall(s) produce a liquid crystal pretilt that combines with the lateral electric field from the edges of the pixel electrode 26 defining the LC cell to cause the LC molecules to tilt in a desired direction when a voltage is applied across the pixel. By providing such tilt control, conventional rubbing steps associated with alignment layers can be avoided. Moreover, the geometry of the wall(s) of the cell may be configured to provide for multi-domains in the given pixel.
A pattern of thin wall(s) or ridges 10 may be employed to form a multi-domain liquid crystal cell through a combination of pre-tilt control and a fringe electric field. This is the approach described in application Ser. No. 08/960,826. Some practical difficulties exist in the alignment of the walls and with the display contrast ratio in this approach. For the case shown in FIG. 4, a ridge 10 is made of a transparent insulating material which serves to control the liquid crystal pretilt for multiple domains. A liquid crystal display 8 includes pixel electrodes 12 and data lines 31 disposed on a thin film transistor glass substrate 15. Ridges 10 are formed on a blanket common electrode 28. Common electrode 28 is formed on a color filter layer 23. A black matrix 23-1 is included which absorbs light to prevent scattering and reflection thereof. A color filter glass or substrate 27 is also present.
For the liquid crystal (LC) molecules near the wall(s) or ridges 10, the slope of the side walls and the alignment layer(s) causes the LC molecules near the wall(s) to tilt in a desired direction either when a voltage is applied across the pixel or when a voltage is not applied across the pixel. For the LC molecules away from the thin wall(s), the slope of the side walls and the alignment layer(s) and the lateral electric field from the edges of the pixel electrode 12 defining the LC cell cause the LC molecules away from the thin wall(s) to tilt in a desired direction when a voltage is applied across the pixel. By providing such tilt control, conventional rubbing steps associated with the alignment layers can be avoided. Moreover, the geometry of the wall(s) of the cell may be configured to provide for multi-domains in the given pixel.
For good viewing angle quality, ridge 10 must be accurately centered on the pixel electrode 12 (space "a" and "b" in FIG. 4 are equal, i.e., symmetrically placed relative to the pixel electrode or features forming the fringe field) and for maximum brightness, the width ("X" in FIG. 4) of ridge 10 should be as small as possible. The display contrast ratio is degraded by light leakage around edges 16 of ridge 10 where liquid crystal 36 is aligned to ridge 10.
Therefore, a need exists for a liquid crystal display structure which minimizes light leakage, provides improved contrast ratio and provides a good viewing angle.